Poly(arylene sulfides) (hereinafter may also be abbreviated as “PASs”) represented by poly(phenylene sulfide) (hereinafter may also be abbreviated as “PPS”) are engineering plastics excellent in heat resistance, chemical resistance, flame retardancy, mechanical strength, electrical properties, dimensional stability, etc. The PASs are commonly used in a wide variety of technical fields such as electrical equipments, electronic equipments, automotive equipments and packaging materials because they can be molded or formed into various kinds of molded or formed products, films, sheets, fibers, etc. by general melt processing processes such as extrusion, injection molding and compression molding.
As a typical production process of a PAS, is known a process in which a sulfur source is reacted with a dihalo-aromatic compound (hereinafter may also be abbreviated as “DHA”) such as p-dichlorobenzene (hereinafter may also be abbreviated as “pDCB”) under heating conditions in an organic amide solvent such as N-methyl-2-pyrrolidone (hereinafter may also be abbreviated as “NMP”). As the sulfur source, is generally used an alkali metal sulfide, an alkali metal hydrosulfide or a mixture thereof. When the alkali metal hydrosulfide is used as the sulfur source, the alkali metal hydrosulfide is used in combination with an alkali metal hydroxide.
The polymerization reaction of the sulfur source with the dihalo-aromatic compound is a desalting polycondensation reaction, and so a great amount of a salt (i.e., an alkali metal halide) such as NaCl is produced after the reaction. In addition, hydrogen sulfide is formed or secondarily produced because of the reaction of the sulfur-containing compound. Therefore, after a PAS formed by the polymerization reaction has been separated and collected from the reaction mixture, the polymer collected has heretofore been washed with a washing liquid such as water, an organic solvent or a mixture of water and an organic solvent to remove the salt such as NaCl and hydrogen sulfide together with the organic amide solvent.
A washing method is roughly divided into batch washing and continuous washing.
The batch washing is a method of washing the polymer collected under stiffing with a washing liquid at a time or in portions for a predetermined period of time in a washing vessel and a method of generally conducting this process several times repeatedly. For example, Patent Literature 1 (Japanese Patent Application Laid-Open No. 6-192423; corresponding to EP Patent Application Laid-Open No. 0594189) has proposed a method of washing the PAS in a washing container and a washing tank. However, one or more washing vessels are required, and so the equipments are complicated, a construction cost is expensive, and increases in the amount of a washing liquid consumed, the amount of a waste liquid treated and stirring power are incurred, whereby increase in a running cost has not been avoided.
On the other hand, there has been proposed, as the continuous washing, a method of bringing a polymer in a wet state into countercurrent contact with an organic solvent or water that is a washing liquid. For example, Patent Literature 2 (International Publication No. 2003/048231; corresponding to US Patent Application Laid-Open No. 2005/0087215) has proposed bringing a polymer slurry containing a polymer into countercurrent contact with a washing liquid by means of a substantially V-shaped pipe device equipped with a downward pipe portion and an upward pipe portion, Patent Literature 3 (Japanese Patent Application Laid-Open No. 3-86287; corresponding to U.S. Pat. No. 5,143,554) has proposed bringing a granular resin of PPS or the like into countercurrent contact with a washing liquid in a pipe body perpendicular provided and having static mixing elements continuously arranged in series. In addition, Patent Literature 4 (Japanese Patent Application Laid-Open No. 2003-516846 through PCT route; corresponding to US Patent Application Laid-Open No. 2003/0150804) has proposed bringing a first liquid phase containing a PAS into countercurrent contact with a second liquid phase such as water while precipitating the PAS under gravity to transfer the PAS to the second liquid phase. Further, Patent Literature 5 (Japanese Patent Application Laid-Open No. 2008-513186 through PCT route; corresponding to US Patent Application Laid-Open No. 2008/0025143) has proposed using a vertical type solid-liquid contact device equipped with a plurality of stirring chambers as a washing device for PAS resin particles and discloses that a downward current of a PAS-containing slurry is brought into countercurrent contact with an upward current of a washing liquid such as water.
Furthermore, it has also heretofore been known to conduct continuous washing repeatedly and conduct batch washing and continuous washing in combination prior to continuous washing.
The continuous washing by bringing the polymer in the wet state into countercurrent contact with the organic solvent or water that is the washing liquid has merits that washing efficiency is high, the equipments are simple, and a running cost is cheap. In particular, the method of bringing the downward current of the PAS particle-containing aqueous slurry into countercurrent contact with the upward current of the washing liquid to discharge the PAS washed and the washing waste liquid from a lower portion and an upper portion of the washing device, respectively, is more efficient because the PAS particles higher in specific gravity than water naturally fall in the washing liquid by virtue of gravity, and the equipments are also simpler. In addition, according to this method, the amount of the PAS particle-containing aqueous slurry supplied and the amount of the washing liquid supplied are controlled, whereby a washing speed can be easily controlled.
The PAS is naturally higher in specific gravity than the washing liquid. However, the precipitation of the PAS particles may be prevented in some cases according to the compositions of the aqueous slurry containing the PAS particles and washing liquid, temperature conditions, etc. In particular, PAS particles relatively small in particle size are hard to be precipitated and may rise toward the level of the washing liquid in some cases. The PAS particles risen in the vicinity of the level of the washing liquid may flow out of a countercurrent washing device in attendance upon the discharge of the washing waste liquid in some cases. In addition, the particle size and particle size distribution of the PAS particles flowed out of the countercurrent washing device in attendance upon the discharge of the washing waste liquid, and the amount of the particles flowed out cannot be surely expected. These PAS particles flowed out partly include those hard to be used in use of PAS products, such as PAS markedly small in particle size. However, many particles thereof may be used in use of products if they are collected. Accordingly, in case that PAS particles flow out of the countercurrent washing device in attendance upon the discharge of the washing waste liquid, their particle sizes and particle size distributions may be different between product lots in some cases, a yield of the PAS product is lowered, and a treatment for cleaning the washing waste liquid is troubled, so that a high-quality PAS cannot be stably produced. Thus, improvement thereof has been required. If the washing waste liquid is filtered through a filter having an extremely small pore size, it goes without saying that the flowing-out of the PAS particles can be prevented. However, the following problems have been unable to be avoided. 1) A high cost is required to re-separate the PAS particles. 2) The filter is clogged if the operation is conducted for a long period of time. 3) Since fine PAS particles are also re-separated, the whole quality of PAS is hard to be maintained and managed to specifications.
In addition, the PAS particles re-separated by filtering the washing waste liquid are markedly contaminated by mixing-in of foreign matters such as metal particles and pieces of a filter medium that is considered to be caused by a separating device including the filter, and so a person skilled in the art has not assumed such particles to be collected as a product.